The Schmitt trigger circuit responds to a slowly changing input waveform to produce fast transitions at the output. The Schmitt trigger is particularly useful in digital circuits, where it may be used to maintain the integrity of the logical one and zero levels. It can also be used with analog systems as a level detector. An important feature of the circuit is that the switching thresholds are different for increasing and decreasing input signals, that is the circuit exhibits hysteresis.
In CMOS, as with other integration technologies, there is a continuing desire to increase the degree of integration to produce more circuits on a wafer of given size. One way in which the number of circuits per unit area may be increased is, of course, to reduce the complexity of circuits.
Known CMOS Schmitt triggers require relatively large numbers (10 or more) of transistors and correspondingly large areas of wafer.